Nobel Prize Recognizes Ultrafast Revolution

The Royal Swedish Academy of Sciences has awarded the 1999 Nobel Prize in chemistry to Ahmed H. Zewail, a professor of chemistry and physics at the California Institute of Technology in Pasadena. The award, which will be formally bestowed on Zewail on Dec. 10, celebrates his development of a technique for ultrafast spectroscopy.

Pioneered by Zewail's team in 1987, the system excites a molecule with femtoseconds-long pulses of laser light and probes it tens of femtoseconds later, enabling researchers to observe the rapid vibrational and electronic changes in a chemical reaction as they occur. Chemists no longer need to infer the fundamental dynamics of complex reactions from the resulting products.

Rudolph A. Marcus, professor of chemistry at Caltech and winner of the 1992 Nobel Prize in chemistry, said that Zewail has presented chemists with a "vivid picture" of the workings of molecules and has given reality to theoretical understandings of matter. "We can look and see bonds breaking and forming as they happen," he said.

Marcus' interests include energy redistribution in vibrationally excited molecules and their decomposition. Zewail's technique could allow Marcus to empirically probe the theory that explains the unexpected, mass-independent behavior of ozone isotopes, for example.

He said there was some understanding of these processes prior to femtosecond chemistry, but that the experimental methods were typically unable to resolve reactions at times less than a picosecond. Chemists could not confirm or refute that understanding. "We can make predictions," he said, "but it's good to have experiments, to make observations."
What is to be learned with faster chemistry?

"We're sort of at the end," said Marcus, explaining that chemical reactions occur on the scale of tens of femtoseconds to years or longer. Chemistry has a limit with nuclear motion, typically on the order of 10 to 20 fs, he said. While shorter pulses could reveal processes of fascination to science in general, such as electronic relaxation, he said, "that's a region of much less interest to chemists."